Interpretive Summary: Soils of the grazing lands in northeastern Australia have only small amounts of nutrients, and depend on plants to capture resources to maintain condition. Heavy grazing threatens to degrade the condition of these lands. During 1998, we examined the combined responses of grass plants, soil microbes and soil nutrients in two sets of pastures after 15 years of no and heavy grazing (applied during 1983-1998). Amounts of microbial carbon, soil carbon and nitrogen, and root surface area in the soil profile were described at different locations from the centers of plants outward to 12 inches (0, ±3, ±6, and ±12 inches) upslope and downslope of plants. We found that microbes, soil carbon and nitrogen in the upper 6 inches of soils were highest in the ungrazed pastures, particularly in the samples closest to plants (0 and ±3 inches). Amounts on heavily grazed pastures were the same across all locations and much lower (20-33%) than on the ungrazed pastures. Root surface area in both types of pastures was not generally found beyond 16 inches from plants or below 15 inches deep in the soil. Levels on the heavily grazed pastures were only 50% of those on ungrazed pastures. In this study, grass plants helped to enrich the soil, but the effect was limited to small areas and was lost under heavy grazing. Results highlight the links that exist between plants, soil nutrients and soil microbes in the grazing lands of northeastern Australia, and point to the importance of balancing grazing pressure against pasture productivity.

Technical Abstract:
Soils in the tropical woodlands of northeastern Australia are nutrient-poor, depend on plants to capture resources to sustain condition, and are threatened by degradation due to heavy grazing. During the dry and wet seasons of 1997 and 1998, we examined the responses of grass tussocks, soil microbes and soil nutrients in sets of paddocks (n=2) following 15 years of no (S1) and heavy (S2) grazing pressures. Soil samples (n=4) were collected from the upper 15 cm of the profile from tussock centers outward to 30 cm (0, ±8, ±15, and ±30 cm) up-and-down slope of the dominant perennial grasses Bothriochloa ewartiana and Chrysopogon fallax. Total soil C and N were determined by wet oxidation and micro-Kjeldahl techniques, respectively, and microbial C was described by conversion of concentration of ninhydrin-reactive N. Root surface area was estimated by scanning samples from cores (to 70 cm depth in 10 cm sections) collected along similar transects. Concentrations of microbial C, soil C and N were present at higher levels in close proximity to tussocks (centers and ±8 cm) on S1 paddocks. Amounts on S2 paddocks were similar across locations and significantly lower (20-33%) than on S1 paddocks. Root surface area on both condition paddocks was mostly located within ±20 cm of tussocks and above 50 cm depth, and amounts were lower on S2 paddocks (50% of S1 paddocks). The localized enrichments near grass tussocks were limited to small areas, and highlight the coupling that exists between plants, nutrient pools, and microbial activity in grazing lands of northeastern Australia. Responses also underscore the importance of balancing grazing pressure against paddock productivity.